Evaporation ducts which extend long-distance coverage and enhance maritime communication quality hold significant potential for 6G broadband communications. The recent three-ray maritime channel model-incorporating rough sea reflections, wave-induced fading, Earth curvature divergence, and evaporation duct trapping-aligns well with high-frequency (5.25-12.74 GHz) measurements but exhibits significant errors at low-frequency (2.02-4.50 GHz) due to unaccounted evaporation duct leakage. To address this issue, a modified model has been developed. Guided by Huygens-Fresnel's principle, the inherent correlation between evaporation duct leakage and electromagnetic wave angular spectrum expansion is analyzed. The parabolic equation method is then employed to quantitatively characterize this correlation, based on which an evaporation duct propagation attenuation factor is derived and further incorporated into a frequency-dependent modified three-ray path loss model. Shore-to-ship measurements verify this model, showing excellent agreement at high frequencies comparable to the latest three-ray model and better agreement at low frequencies, demonstrating superior predictive performance.

In current vortex electromagnetic wave (VEMW) imaging, multipath effects are often overlooked, severely hindering low-altitude target detection and tracking. Addressing this limitation, this letter establishes a rigorous analytical framework for VEMW multipath propagation. We explicitly derive closed-form expressions for direct and reflected waves, revealing the deterministic topological charge inversion upon surface reflection. Based on this mechanism, we quantify the imaging coordinates, uncovering the distinct spatial heterotopy between aligned real targets and divergent multipath ghosts across VEMW multiple-input single-output (VEMW-MISO) and multiple-input multiple-output (VEMW-MIMO) systems. Exploiting this spatial decoupling, we propose a zero-hardware-cost incoherent fusion strategy to robustly suppress multipath artifacts, and further demonstrate the VEMW-MIMO system's inherent anti-deep-fading capability. Numerical simulations validate both the artifact suppression and anti-fading performance, offering a practical solution for VEMW-based low-altitude imaging in multipath environments.

This letter proposes a novel model for inverse synthetic aperture radar (ISAR) imaging of low-angle targets in the troposphere using echoes simulated by the ray tracing method (RTM). The proposed approach integrates the electromagnetic (EM) wave propagation model with radar imaging techniques to account for the influence of environmental factors, such as atmospheric refraction and ground multipath, on ISAR imaging. First, a spherical multipath calculation method based on the RTM is proposed to acquire path parameters. Subsequently, an environmental transfer function, incorporating path parameters, antenna pattern, and surface characteristics, is created and combined with the target reflectivity function to generate the echo data matrix. Range-Doppler (RD) processing is then applied to obtain the target's ISAR image. Finally, a typical example is given to validate the correctness and effectiveness of the multipath calculation method. Meanwhile, simulation analysis of ISAR images for an aircraft in flight in a tropospheric environment is conducted. Results indicate that the proposed method effectively predicts the impact of the tropospheric environment on ISAR imaging of low-angle targets.

This paper presents a novel synthesis method for orbital angular momentum (OAM) rectangular array patterns. By leveraging the angular symmetry properties of OAM arrays, we simplify the two-dimensional array into an equivalent one-dimensional array with elements that have a Bessel function radiation pattern. Convex optimization is employed to determine the feeding weights of the linear array while adhering to constraints on first null and sidelobe levels. Subsequently, the feeding excitation of the planar array is computed using weighted interpolation, combined with OAM phase modulation to generate radiation patterns for various OAM modes. Numerical examples demonstrate that the first null beamwidth for different OAM modes is approximately 15°, with sidelobe levels below -30 dB. The method shows strong adaptability to large-scale arrays and holds potential for advancing electromagnetic vortex imaging techniques and OAM multimode multiplexing communication.

To address the challenge of predicting radar signal path loss in complex nearshore wind farm environments, this letter proposes a hybrid method combining the pseudo-3D parabolic equation and Fresnel-Kirchhoff diffraction theory. The pseudo-3D parabolic equation method is used to compute environmental path loss by incorporating complex geographical and meteorological conditions into the electromagnetic model. To account for the additional diffraction effects introduced by wind turbines, a supplementary model based on Fresnel-Kirchhoff diffraction theory is integrated. This combined approach enables accurate and comprehensive path loss prediction across the entire wind farm environment. To validate the proposed method, a numerical example under a simplified scenario was conducted and compared with results from the 3D parabolic equation method. The proposed method significantly improved computational efficiency while maintaining a root mean square error below 0.63 dB, demonstrating its high efficiency and accuracy. Further numerical simulations were conducted in a nearshore environment using digital maps, aiming to analyze the impact of wind farms on radio wave propagation from shore-based radar. The results indicate the presence of a pronounced fan-shaped affected zone behind the wind farm, within which the environmental path loss exhibits significant spatial nonuniformity.

Time-modulated arrays (TMAs) have a high design degrees of freedom (DoFs) to improve radiation performance, while they are prone to failure due to their hardware characteristics. In this article, we propose a novel technique to diagnose impaired TMAs based on compressed sensing (CS). The TMA diagnosis problem is reformulated as a sparse signal recovery problem at the center frequency and sidebands. Then, a method based on the difference of convex sets theory and sequential convex programming (DCS-SCP) is developed to implement diagnosis for impaired TMAs. Using a small number of far-field measurements at the same position but different frequencies, the joint recovery of the equivalent excitations at the center frequency and sidebands is realized by a mixed l(0)/l(2)-norm minimization method. The numerical simulation and the successful comparison with the state-of-the-art algorithms demonstrate the superiority of the proposed methods in terms of noise robustness and diagnosis accuracy.

Orbital angular momentum(OAM) communication requires the alignment of the transmitting and receiving antennas, which limits the application scenarios of OAM communication. In this letter, we propose an OAM mode modulation-based wireless backhaul communication scheme for the downlink in a broadcast network. According to the principle of superposition of electric fields, this scheme generates multiple OAM modes simultaneously through uniform circular array(UCA) for information modulation and radiates signals to the outside in a periodic cyclic excitation manner. This scheme does not require the alignment of transmitting and receiving antennas, and the deployment of the receiving antenna position can be very flexible. Compared with QAM modulation, OAM mode modulation has strong anti-interference performance. Meanwhile, as the modulation order increases, the bit error rate(BER) will not change significantly.

Radio environment map (REM) is a crucial part of the cognitive radio system. In this letter, a multisubgrid (MSG) partition strategy based on geographic and geomorphologic characteristics for adaptive refinement is proposed. Combined with cylindrical parabolic equation model (CPEM), the proposed strategy can calculate local radiation distribution in a complex environment with high accuracy and efficiency. Local simulation results are mapped onto the global fine grid (FG) to generate REM in a large-scale region. REMs in the same maritime environment are simulated, respectively, by fine-grid-CPEM (FG-CPEM), coarse-grid-CPEM (CG-CPEM), and multisubgrid-CPEM (MSG-CPEM). Simulation results show that MSG-CPEM outperforms the CG-CPEM in terms of root mean square error and archives higher compute efficiency than that of FG-CPEM. It balances the accuracy and efficiency of the calculation for REM in a large-scale region with complicated geographic and geomorphologic characteristics.

A four-port shared-aperture antenna with a multifunction metasurface is proposed and analyzed in this paper. The adopted metasurface can be simultaneously used as the radiation sources of a S-band microstrip patch radiator and a Ka-band microstrip grid array (MGA) and as the partially reflective surfaces (PRSs) of a X- and a Ka-band Fabry-Perot (F-P) antennas. By integrating the above four radiators compactly, the final antenna operates at 2.3, 11.5, 26.0 and 40.0 GHz each with their bandwidths of 4.0%, 2.0%, 7.4% and 9.5%. Besides, within these operation bands, the realized peak gains are 7.1, 11.0, 15.2 and 19.2 dBi, respectively.

This paper extends previous work on electromagnetic (EM) reciprocity theorem analysis from the single-conductor to the more general multiconductor transmission line (MTL) case. The focus is on the EM coupling problem of an ideal MTL above a perfectly electrically conducting ground when excited by a plane wave. The proposed methodology addresses mutual coupling effects and mismatched loads in the MTL. The expression for the current along the MTL in the test (radiation) state is derived, considering these factors. A novel calculation method for the induced voltage at the load is then introduced, assuming consistent coupling between different lines in both test and receiving states. Numerical examples validate the proposed EM-reciprocity method, offering an alternative approach for analyzing the field-to-line coupling problem in MTL model.

In this letter, we propose a novel 3-D hybrid approach for predicting the electromagnetic (EM) scattering from electrically large targets within ducting maritime environments. The approach integrates the 3-D parabolic equation (PE) method with the multilevel fast multipole algorithm (MLFMA), addressing separately the propagation of EM waves and the scattering by the targets. In addition, a data transfer scheme based on the discrete Fourier transform form of the 3-D PE is proposed to achieve a nearly lossless transition from the 3-D PE to the MLFMA. This scheme not only eliminates the need for local refinement of the PE grid but also avoids the interpolation approximation errors common in traditional interpolation-based hybrid schemes. Finally, a calm sea environment case is provided to validate the accuracy and effectiveness of the proposed method. Meanwhile, the EM scattering from a missile target under several representative cases within a surface ducting maritime environment is calculated and discussed.

The different modes of orbital angular momentum (OAM) beams typically exhibit varying divergence angles, which pose significant challenges for the system design at the receiver end. In response to this issue, the paper proposes a pattern synthesis method for multiple modes OAM uniform circular array (UCA) with equal-divergence angle. Using a Bessel function approximation of the OAM-UCA array factor, the paper fits an analytical expression representing the relationship between OAM beam modes, divergence angles, and UCA radius. It is used to determine the radii of individual UCAs under the constraint of equal divergence angles. The radii are embedded in the calculation of multi-layer concentric ring array factors, and combined with the principle of pattern multiplication, the multimode and equal divergence angle pattern synthesis of OAM-UCA array is realized. The simulation results indicate that the method proposed in this paper efficiently and rapidly achieves the synthesis design of the direction pattern for a UCA with three OAM modes, each having a divergence angle of 18.25 degrees. A comparative analysis with existing literature, including full-wave simulations and experimental results, verifies the effectiveness and superiority of the proposed method.

This letter explores the possibility of long-distance transmission of an orbital angular momentum (OAM) beam through rectangular tunnels. Theoretical analysis reveals that the OAM beam propagating in the tunnel with square cross section fulfills the conditions for axial propagation and exhibits azimuth symmetry. In comparison with free-space OAM generation, we draw the conclusion that long-distance transmission of OAM beams is achievable in the tunnels with square cross section. The validity of our conclusion is further substantiated through numerical experiments. Simulation results demonstrate that the first-order OAM beam radiated by a uniform circular antenna array exhibits favorable helical phase distribution and the circular symmetry of the amplitude distribution in the tunnel beyond the Rayleigh distance. However, these characteristics degrade with increasing mode order and propagation distance, because the increase of high-order OAM beam divergence angle and propagation distance will exacerbate the impact of multipath effects in the tunnel environment.

This article describes an encoding method for the orbital angular momentum(OAM) mode used in communication. Based on the mutual orthogonal characteristics between different mode OAM beams and the principle of electric field superposition, the OAM beams of different modes are superimposed. The number of OAM modes corresponds to a binary sequence of the same length, and each mode represents a data bit, thereby achieving OAM mode encoding. This article designs an 8-element uniform circular array(UCA) that can generate -3 to +3 OAM modes, encoding a 6-bit binary sequence. The coaxial transmission of seven representative multimodal OAM beams is simulated using the parabolic equation(PE) method, and mode analysis is performed to verify the correctness of the scheme.Adding Gaussian white noise to the OAM beam can make the bit error rate(EBR) as low as 0 even when the signal-to-noise ratio(SNR) is -10dB and an appropriate decision threshold is adopted. It reflects the excellent anti-noise performance of the encoding system.

In this paper, an extended matrix pencil method (EMPM) is proposed to achieve the equivalent analysis of a multiport system. First, the Hankel matrix constructed by the sampling data from multiple ports is integrated into an extended matrix. Then, operations, including singular value decomposition, order reduction, and solution of matrix pencil, are carried out for the extended matrix to calculate the common poles. Further, the residue matrix of the multiport system is solved by the least square method. Furthermore, for the multiconductor transmission line (MTL) system containing the frequency-dependent network (FDN) loads, a combination of the EMPM and piecewise linear recursive convolution (PLRC) is proposed to complete its transient analysis. The corresponding voltage expressions at the connection point of the MTL and FDN are analytically derived. Finally, the effectiveness of the EMPM in analyzing the FDN equivalent and that of the proposed combination of the EMPM and PLRC in analyzing the transient response of the MTL system containing the FDN are verified by numerical examples.

To overcome the defect that Two-dimensional Parabolic Equation(2DPE)can only model two-dimensional communication links without considering lateral diffraction and backward reflection of electromagnetic waves near buildings in urban environment,a modified 2DPE model based on Deep Neural Network(DNN)is proposed.The three-dimensional model of urban buildings is built through digital elevation map,from which seven characteristics such as propagation distance,propagation angle,and building coverage are extracted to characterize the distribution of buildings on the propagation path and the deployment of transceiver antennas.Then combined with the measured data,the dataset for correcting 2DPE is constructed;through the training of DNN,the modified model of 2DPE is constructed to make it suitable for the prediction of radio wave propagation in a complicated three-dimensional environment.The simulation results show that compared to linear regression,support vector regression,and decision tree model,the calculation accuracy of the 2DPE correction model based on DNN is high in the three-dimensional propagation environment,and the prediction error on the test set is reduced by a maximum of 46.8%.

A rapid solution for evaluating the radiation pattern of aperiodic arrays, taking into account mutual coupling, is presented in this paper. The evaluation is achieved by eliminating the anisotropy of the active element pattern of the array through the use of the mutual coupling compensation matrix (MCCM) technique, in conjunction with the non-uniform fast Fourier transform (NuFFT). In order to eliminate the impact of mutual coupling on array pattern calculation, the MCCM is utilized to convert the active element pattern (AEP) of each element into a shared uniform term and make the NuFFT technique suitable for the array pattern calculation. The proposed solution is validated by evaluating the radiation pattern of a 64-element planar aperiodic array. In addition, the proposed solution is integrated into the Particle Swarm Optimization (PSO) to realize a pattern synthesis method. Two synthesized patterns, including a pencil beam with low sidelobe level and a flat-top beam pattern, are executed to validate. Compared with several reported methods, the proposed method can improve the synthesis efficiency and maintain good accuracy simultaneously.

To model the radio wave propagation within channels of Backscatter Communication (BackCom) systems with Intelligent Reflecting Surface (IRS) included, an efficient hybrid method based on the Parabolic Equation (PE) method and Method of Moment (MoM) is proposed in this paper. The propagation modeling of IRS-assisted channels in electrically-large scenarios is considered in this method through aspects of radio wave propagation and electromagnetic scattering. The two aspects are then numerically solved by the PE method and MoM, respectively. Through simulations of IRS-assisted channels in line-of-sight as well as non-line-of-sight scenario, the efficiency of the PE-MoM hybrid method is demonstrated. Simulation results show that the computational speed of the proposed algorithm is 6.46 times faster than that of MoM. Meanwhile, the computational resource consumption is also reduced by 81% with the relative root mean square error maintained as 3.89%. The comparison of results shows that the proposed PE-MoM hybrid method can realize the propagation simulation of the IRS-assisted BackCom channels with a better tradeoff between the computational accuracy and computational efficiency achieved.

雷达回波信号的获取对研究动态目标识别和雷达成像具有十分重要的意义.提出了一种基于时域弹跳射线法(Time Domain Shooting and Bouncing Ray,TDSBR)的动态目标回波仿真模型,用于模拟海上电大目标的近场雷达回波.该模型采用TDSBR方法直接在时域计算弹载雷达照射下的目标与海面近场复合散射特性,并结合"stop-go"方法模拟雷达与目标之间的相对运动,实现了弹载雷达照射下海上运动舰船目标的回波信号仿真.仿真结果表明,该模型可以有效呈现海面与海上目标的耦合效应,实现对目标速度和距离的预测,具有良好的精度和较高的效率,在海洋环境中弹载雷达的回波预测和目标检测中具有广阔的应用前景.

This letter proposes a 3-D parabolic equation-method of moments (PE-MoM) hybrid method to predict radar target scattering with surface ducting in large-scale environments. The method divides the complex multiscale problem into two subproblems: electromagnetic (EM) wave propagation and scattering. The bidirectional 3-D parabolic equation method is used to consider the influence of environmental factors, such as irregular terrain and atmospheric duct on EM wave propagation, whereas the method of moments (MoM) is used to calculate the scattering from the key target. By introducing phase compensation and Fourier synthesis techniques, the proposed method can be extended to predict transient scattering echoes from the target. A typical example is given to verify the correctness of the proposed method, and good consistency is also seen from comparison with the MoM. Finally, simulation analysis is carried out for the scattering of a missile target in a sea–land composite environment. The results show that the proposed method can effectively predict the scattering echoes of low-altitude targets in large-scale environments.